1. Field of the Invention
The present invention relates to an optical film, a process of producing it, and a polarizing plate and an image display device employing it.
2. Description of the Related Art
Heretofore, a diffuser sheet is used in various image display devices. For example, in a liquid crystal display device (LCD), in general, a diffuser sheet is disposed between the backlight source and the backlight-side polarizing plate therein. Thus disposed in such a device, the diffuser sheet contributes toward unification of display characteristics, and acts to prevent formation of interference fringes such as moire to be caused by incident light that interferes with pixels in a liquid crystal cell. These days for the purpose of reducing the production costs, reducing the number of the members to constitute a liquid crystal display device is tried, and for energy-saving, reducing the number of the fluorescent lamps to be used as a light source is also tried. In addition, for further reducing the body thickness of LCD, the distance between the backlight source and the diffuser sheet in the device is reduced, and as a result, a conventional diffuser film could hardly attain uniform light diffusion.
For example, JP-A 2000-75134 proposes a light-diffusive polarizing plate having a light-diffusive layer, which has predetermined characteristics and which contains porous amorphous particles and spherical particles as dispersed therein, and this discloses the capability of omitting a light diffuser sheet. JP-A 2001-172403 proposes a method for producing a light diffuser film by casting a dope, containing particles, onto a support, and discloses the capability of producing a light diffuser sheet excellent in optical anisotropy according to this method. JP-A 2001-226501 proposes a method for producing a light diffuser film by stretching a polyester film containing a phase separation component, and discloses simple production of a light-diffusive resin film according to the method. JP-A 2001-4813 proposes a method for producing a light diffuser film by through stretching with formation of internal bubbles. JP-A 9-155981 discloses a film with internal bubbles formed therein, but has no description relating to the detailed structure of the bubbles in the film. On the other hand, JP-A 2003-29030 discloses a film having fine pores of specific morphology, in which, however, the production method for it is a special method. In JP-A 2003-29030, improving only the transmittance and the scatterability of the liquid crystal display device comprising an anisotropic scattering film produced therein is specifically noted, and nothing relating to a technique of unifying the display quality is not under investigation.